The invention relates to an improved port circuit for token ring networks and to networks and methods employing the improved port circuit.
Local area networks are used to transmit digital data between electronic devices, e.g., data processors such as computers. In a token ring network, the electronic devices are connected to the network at stations that are serially connected by a transmission medium in an endless logical loop, i.e., the ring. The data are transmitted serially in one direction from station to station along the ring, each station receiving the data at its receiver line and repeating or transmitting the data at its transmitter line. A station that obtains access to the network places data on the network addressed for another station. The addressed station copies the information as it passes through it while the nonaddressed stations merely repeat the data and send them along the ring. When the data return to the sending station, they are removed from the ring. Access to the network for placing data on it is controlled by a "token", a unique bit sequence, that is transmitted along the ring. A station obtaining access to the network modifies the token before sending data and restores the token after it has completed sending data, permitting another station to then seize the token and send data.
Token ring networks are often wired in a so-called "star" configuration (e.g., as specified in ANSI/IEEE Standard 802.5), with a concentrator (also referred to as a "hub") at the center and paired transmitter and receiver wires going out from a respective port circuit of the concentrator to each station. Each port circuit has a path switch. The path switches are wired sequentially in a loop at the concentrator. Each path switch can by-pass its station's transmitter and receiver wires (in which case the station is not part of the ring) or connect the receiver and transmitter wires to the upstream and downstream nodes of the path switch (in which case the station is part of the ring and can transmit, receive, and repeat data passed along the ring). Some concentrators, known as active concentrators, have port circuits with amplifiers and equalizers to reduce the effect of attenuation of the signal. Concentrators that do not include such amplifiers and equalizers are known as passive concentrators. Some port circuits have included timing recovery circuits for data received from a station in addition to amplifiers and equalizers.